#!/usr/bin/env perl                                                                             

use strict;
use warnings;

undef $/;

my %vars = ( name  => qq(Karl),
             color => qq(blue),
             fruit => qq(tomato)
           );

my $string = <DATA>;

$string =~ s/%%(.*?)%%/$vars{$1}/gs;

print $string;

__DATA__
Hello, my name is %%name%%!                                                                     
May i have a %%color%% %%fruit%% please?